TIL: Very useful Linux/Unix commands

Here is a list of useful unix commands or code parts. Who does not know it? You have a problem and looking for a solution where you find at stack overflow or similar pages? Here I collect all the commands that I have encountered over time or whose switch I simply can not remember (or want).

How do I find all files containing specific text?
```
grep -rnw '/path/to/somewhere/' -e 'pattern'
```
How i change the default file permissions (mask that controls file permissions)
```
umask
```
Untar (unzip) file/folder
```
tar -zxvf archive.tar.gz
```
Tar (zip) file/folders
```
tar -cvzf archive.tar.gz file1 file2
```

Copy files via rsync from one host to another

rsync -avz [USER@]HOST:SOURCE [USER@]HOST:DEST
rsync -avz [USER@]HOST:SOURCE rsync://[USER@]HOST[:PORT]/DEST
rsync -avz -e "ssh -p 12345" LOCAL/SOURCE [USER@]HOST:DEST

Using rsync with sudo on the destination machine
1. Find out the path to rsync: which rsync
2. Edit the /etc/sudoers file: sudo visudo
3. Add the line <username> ALL=NOPASSWD:<path to rsync>, where username is the login name of the user that rsync will use to log on. That user must be able to use sudo

Then, on the source machine, specify that sudo rsync shall be used:

rsync -avz --rsync-path="sudo rsync" SOURCE [USER@]HOST:DEST

Preserve SSH_AUTH_SOCK (Environment Variables) When Using sudo
```
sudo --preserve-env=SSH_AUTH_SOCK -s
```
nslookup missing? Install dig
```
sudo apt-get install dnsutils
```

find without "Permission denied"

find / -name 'filename.ext' 2>&1 | grep -v "Permission denied"

flush dns cache
```
sudo systemd-resolve --flush-caches
```
show open ports
```
netstat -tulpn
```

Directory size

du -sh /var
du -shc /var/*
du -h --max-depth=1 /var
du -sh /var/lib/docker/containers/*/*.log

Search multiple PDF files for a "needle"
```
pdfgrep -i needle haystack*.pdf
```
Show hidden files with ls
```
ls -lar
```
Redirect STDOUT and STDERR to a file
```
nice-command > out.txt 2>&1
```
Installs your SSH public key to a remote host
```
sh-copy-id 'user@remotehost'
```
A command-line system information tool
```
neofetch
```
Show disk usage, folder size, items per folder, find big directorys, ... with ncdu
```
ncdu
```
Fancy resource monitor
```
btop
```
Display disk activity
```
iotop
```
Display network activity
```
iftop or iptraf
```
Cleanup Docker
```
docker system prune --help
```
Find and repair disk errors on ext (ext2, ext3 and ext4) filesystems
```
sudo e2fsck -f </dev/sda2> 
```

Forward TCP/UDP ports with socat

socat TCP-LISTEN:8080,fork,reuseaddr TCP:homeserver.local:8080

Traceroute and ping in one, visual console tool
```
mtr <host>
```

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